Carrier billing lets you buy services or apps and have the cost added directly...

https://wiki-mixer.win/index.php/What_Is_the_Simplest_Explanation_of_Carrier_Billing_for_a_Friend%3F

Carrier billing lets you buy services or apps and have the cost added directly to your monthly phone bill or taken from your prepaid credit. It’s a simple way to pay without needing a credit card